In single mode the current working directory is added to import paths by default

[drug007]

Fix #2051

This PR adds the current working dir to the list of import paths if user run test command in single file mode like:

dub test --single path/to/file.d

to let dub to find the file.

Important:
the file should not be named main.d because the file is imported as module main and there will be conflict between static import main and function D main:

/tmp/dub_test_root_test.d(9,12): Error: function D main conflicts with static import dub_test_root.main at /tmp/dub_test_root_test.d(3,15)

[dlang-bot]

Thanks for your pull request and interest in making D better, @drug007! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

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[thewilsonator]

Please add a changelog entry and a test case.

[drug007]

Closed as obsolete, see #2052